The present invention is related to capacity modulation of compressors and more particularly to capacity modulation of scroll-type compressors.
Capacity modulation is often a desirable feature to incorporate in air conditioning and refrigeration compressors in order to better accommodate the wide range of loading to which the systems may be subjected. Many different approaches have been utilized for providing this capacity modulation feature ranging from controlling of the suction inlet to bypassing discharge gas back to the suction inlet. With scroll-type compressors, capacity modulation has often been accomplished via a delayed suction approach which comprises providing ports at various positions which, when opened, allow the compression chambers formed between the intermeshing scroll wraps to communicate with the suction gas supply thereby delaying the point at which compression of the suction gas begins. This method of capacity modulation actually reduces the compression ratio of the compressor. While such systems are effective at reducing the capacity of the compressor, they are only able to provide a predetermined amount of compressor unloading, the amount of unloading being dependent upon the positioning of the unloading ports along the wraps. While it is possible to provide multiple step unloading by incorporating a plurality of such ports at different locations, this approach becomes costly and requires additional space to accommodate the separate controls for opening and closing each set of ports.
The present invention, however, overcomes these deficiencies in that it enables virtually a continuous range of unloading from 100 percent or full capacity down to virtually zero capacity utilizing only a single set of controls. Further, the system of the present invention enables the operating efficiency of the compressor and/or refrigeration system to be maximized for any degree of compressor unloading desired.
In the present invention, compressor unloading is accomplished by cyclically effecting axial or radial separation of the two scroll members for predetermined periods of time during the operating cycle of the compressor. More specifically, the present invention provides an arrangement wherein one scroll member is moved axially or radially toward and away from the other scroll member in a pulsed fashion to cyclically provide a leakage path across the tips or flanks of the wraps from higher pressure compression pockets defined by the intermeshing scroll wraps to lower pressure pockets and ultimately back to suction. By controlling the relative time between sealing and unsealing of the scroll wrap tips or flanks, virtually any degree of compressor unloading can be achieved with a single control system. Further, by sensing various conditions within the refrigeration system, the duration of compressor loading and unloading for each cycle can be selected for a given capacity such that overall system efficiency is maximized. For example, if it is desired to operate the compressor at 50 percent capacity, this can be accomplished by operating the compressor alternately in a loaded condition for five seconds and unloaded for five seconds or loaded for seven seconds and unloaded for seven seconds, one or the other of which may provide greater efficiency for the specific operating conditions being encountered.
The various embodiments of the present invention described below provide a wide variety of arrangements by which one scroll member may be axially or radially reciprocated with respect to the other to accommodate a full range of compressor unloading. The ability to provide a full range of capacity modulation with a single control system as well as the ability to select the duration of loaded and unloaded operation cooperate to provide an extremely efficient system at a relatively low cost.
Additionally, in order to even further improve system efficiency in some applications, it may be desirable to combine a delayed suction type of capacity modulation with the pulsed unloading approach mentioned above. For example, when operating conditions are such that system pressures just downstream of the discharge valve are at a level below the full load design level, the compression ratio of the compressor will result in pressure of the compressed fluid as it is discharged from the compression chamber being too high, a condition known as over-compression. The most efficient way to reduce capacity under these conditions is to reduce the compression ratio of the compressor and hence the pressure of the compressed fluid exiting the compression chamber such that it is equal to or only slightly above the system pressure just downstream of the discharge valve thus eliminating the lost work due to over-compression. However, if a further reduction in capacity is indicated by system condition once the over-compression condition has been eliminated, the use of a pulsed type of capacity modulation will be more efficient as it will avoid creation of a condition known as under-compression, that being a situation where the pressure of the compressed fluid as it leaves the compression chamber being below that of the system just downstream of the discharge valve. Thus, the present invention also includes a system in which both pulsed and delayed suction capacity modulation approaches are combined which result in even greater efficiencies for systems likely to encounter such operating conditions than could be achieved by either of the two capacity modulation approaches alone.
Additionally, the present invention may also incorporate a motor control module which will operate to control various operating parameters thereof to enhance its operating efficiency during periods when the motor load is reduced due to unloading of the compressor.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.